Nobody's mentioned Casey Candaele. Maybe not the best, but in the conversation. If Milo labels you the "Mighty Mite", you must be doing something right. I'm not sure that some of the guys mentioned like Walling and Craig Reynolds should qualify. Walling spent several years as a full time 3rd baseman, and Reynolds did the same as a full time SS.
G Rat mentioned him, but he deserves a second. He played all 9 positions in 91, I think. Can't really top that unless you sub in for the manager or something.
Eric Brunlett's fielding was good enough to be a starter anywhere in the infield. He also knew how to run the bases pretty well, it was just the hitting that kept him from a starting spot.